feat: menu name may use string field

[hai-vr]

This is a proposal to include a new "label" field in Modular Avatar Menu Item.

Although not specified, menu items may contain line breaks and rich text, which will be displayed in the radial menu.

It is currently really awkward to write rich text within the object name. In addition to being long, rich text tags will contain a forward slash / and other symbols.

Exposing that field may enable other inventory automation tools to more precisely control the generation of the expression menu items.

This PR attempts to add the following:

• Adds a new serializable field "label" in the Modular Avatar Menu Item.
• When this "label" field is not an empty string, then Modular Avatar Menu Item would use that instead of the GameObject name.
• In the UI, add some means to enable that field for editing, add a multi-line field for that label, and a rich text preview area.

The UI method is currently unsatisfactory, I do not know what would be a good fit. Most users will not use rich text, so the current proposal hides the existence of that new field to the user unless they click a "Rich text" checkbox, which temporarily enables the visibility of the field even when that field is an empty string.